1. Field of the Invention
The present invention generally relates to pipeline installation and, more particularly, is concerned with a trenchless method and for accurate installation of a pipeline in which corrective alignment of an initial inaccurate pilot hole is carried out before pipeline installation take place.
2. Description of the Prior Art
In the traditional way of laying a new underground pipeline system, a deep trench is normally first dug along the route planned for the pipeline system. Next, the pipeline is laid in the trench. Then, after the pipeline is so laid, the trench is filled in and the original surface restored. Significant disruption to neighborhoods, traffic flow, business operations and public convenience is experienced by this traditional trenching method of laying a new underground pipeline.
In the replacement of old underground pipelines with new pipelines, a trenchless method has been developed which substantially avoids having to dig up and then restore the surface. The trenchless pipeline replacement method utilizes the old pipeline to be replaced as a pilot hole for guiding an installation tool for either bursting or extracting the old pipeline and installing the new pipeline in its place. At most, the only excavation necessary is the digging of spaced pits for insertion (and, in some instances, extraction) of pipeline, disconnection and reconnection of adjacent service lines and placement of boring drive machines. As an example, when replacing an existing watermain on a typical residential block about 150 meters in length, the total length of the pipeline route which has to be excavated for the machine, insertion/extraction and service pits is around 20 to 25 meters. This leaves about 85% of the pavement along the pipeline route undisturbed.
These two principal trenchless methods are suited primarily for pipeline replacement in which the old pipeline laid by the trenching method is used as the pilot or guide hole. Various approaches have been devised to apply the trenchless method to the laying of a new, original (as opposed to replacement) pipeline system. These approaches, commonly known as directional drilling, rod pushing, air piercing and microtunneling, involve placing a pilot hole insertion machine either on the surface or in an entry pit to drive an insertion tool to produce a pilot hole from the entry pit to the exit pit. After reaching the exit pit, the pilot hole insertion tool is retracted from the pilot hole while a string of drive rods from a drive unit are advanced along the pilot hole to the entry pit. Once the drive rod string reaches the entry pit, an enlargement tool is attached to the drive rod string and the tool is pulled back to the exit pit, opening up the pilot hole and installing the new pipeline behind it, as in the pipe bursting or pipe extraction method of trenchless pipeline replacement.
However, a serious drawback of the aforementioned approaches is that the conventional machines typically used are unable to produce the pilot hole with the accuracy needed for the centerline of the pilot hole to meet grade requirement for gravity flow distribution systems, such as sewerlines. Thus, in new construction projects, the more costly, traditional trenching method typically must be used.
Consequently, a need exists for improvement of the aforementioned conventional approaches so as to overcome the drawback described above with respect to the lack of accuracy in producing the pilot hole.